Direct electronic fourier transforms of optical images

ABSTRACT

Method and apparatus for directly converting between optical images and the spatial Fourier transforms of optical images by interacting sound waves and light. Controlled sound waves couple with optical images, and electrical signals may be derived from this coupling which are functions of the spatial Fourier transforms of the entire optical images. In a reverse process, optical images are obtained directly by coupling controlled sound waves with electrical signals which are a function of the spatial Fourier transforms of the optical images and with light.

United States Patent [1 1 Kornreich et al.

DIRECT ELECTRONIC FOURIER TRANSFORMS OF OPTICAL IMAGES Inventors:Philipp G. Kornreich, 5548 Bear Rd.. North Syracuse.N.Y. 13212: StephenT. Kowel, 1 1 1 Lafayette Rd..Syracuse.N.Y. 13205 Filed: May 30, 1973Appl. No.: 365,054

Related US. Application Data Division of Ser. No. 319,680, Dec. 29,1972.

US. Cl 178/71, 178/73, 178/76,

178/54 BD Int. Cl. H04n 5/30 Field of Search l78/7.l, 7.3, 7.6, 5.4 R,

References Cited UNITED STATES PATENTS l/l971 Cook l78/7.l

[451 Sept. 17, 1974 3,588,324 6/1971 Marie 178/54 BD 3,617,931 11/1971Pinnow et al. 178/7.6 3,633,996 1/1972 Lean et a1 178/76 PrimaryExaminerRichard Murray ABSTRACT 36 Claims, 12 Drawing Figures F 1 LIGHTPHASE P4 TTERA/ swvcmauous A PROJECTOR DETECTOR 34 m 32 co/vsm/vrELECTRICAL 2521??? SWEEP cds FILM COA'MCU FREQUEUCV GtA/ERATOR IPAZAgDUCFR REED Q04? 72 suasmn re ACOUST/CAL ABSOAB/NG T41 6 Pmmmsw m sum 20r 6 Z 1 I l REG/9N2 1.-- REG/0A! I m 1 LIGHT PHASE P4 TTERA/ sYA/c/mwous pm/5c T0,? DETECTOR 34 24 am 2 ELECTRICAL AMI; sougf SWEEPeds FILM GOA/T4675 FREQUENCY saw/94m f2 fPAMfDUCE/x FUSED Q UAR TZ SUBS7R4 TE ACOUST/CAL ABSORBING v TAPE 1 DIRECT ELECTRONIC FouRIERTRANSFORMS or OPTICAL IMAGES This is a division of application Ser. No.319,680 filed Dec. 29, 1972.

BACKGROUND OF THE INVENTION The invention herein described was made inthe course of or under a contract or subcontract therein with theDepartment ofthgAirforgg The invention relates to converting pictorialinformation into electrical signals and to converting electrical signalsinto pictorial information. More specifically, the invention relates toobtaining Fourier transform representations of pictorial information,and for converting such Fourier transform representations into thecorresponding pictorial information. Still more specifically, theinvention relates to directly converting between optical images and theFourier transform representations of the images.

Electronic processing of pictorial information is an active field, andthere are tnany devices for converting between pictorial information andelectrical representations thereof. Such prior art devices commonlyrequire arrays of small photosensitive elements. The individual elementsof an array are sensed for changes in a photosensitive parameter when anoptical image is incident on the array. This is explicitly the case indevices such as photodiode mosaics, and is implicitly the case withdevices such as the Vidicon tube, where only a small region of thephotosensitive surface contributes at any one time to the Video signalderived from the tube. In such prior art devices, the instantaneousvalue of the derived electrical signal generally represents the lightintensity of a particular portion of an image. Such electrical signalsmay be later processed, such as by analog or digital computers, into aFourier transform representation of the signals and hence of the imagerepresented by the signals. The Fourier transform representation isdesirable, because it allows for more efficient and more versatileelectronic processing of images,

such as for improving image resolution, removing noise, providingelectronic zoom operations, motion and speed detection, patternrecognitions, bandwidth compression, etc.

The advantages associated with the use of Fourier transformrepresentations of pictorial information have led to many devices forobtaining such representations. For example, there are programs forutilizing general purpose digital computers to obtain the Fouriertransform representation of electrical signals, and there is a A classof special purpose machines called Fast Fourier Transform Computers.Additionally, there is a laser technique for optically obtaining theFourier transform of laser images. This laser technique is based on theobservation that a planar density pattern of coherent light gives riseto its Fourier transform when the pattern is placed in the front focalplane of a lens and the result is observed in the back focal plane (see,for example, Poppelbaum, Computer Hardware theory, McMillan, 1972, pages626 et seq. It is emphasized that this laser technique is limited tousing coherent light, and cannot be extended to conventional pictorialinformation which, of course, is a non-coherent and polychromaticoptical image.

Because of the desirability of having Fourier transform representationsof pictorial information, there is a need to obtain such representationssimply and efficiently.

It is known that there are relationships between mechanical deformationsof certain materials, optical images incident on the materials andelectrical signals associated with the materials. One example of adevice utilizing such relationships is disclosed in U.S. Pat. No.3,202,824 issued to Yando in 1965. The patent relates to a pick-updevice employing a photoconductive layer in which a light patternprojected on the layer is transformed into a series of output voltagepulses. These output pulses are produced by propagating an elastic waveaccompanied by an electric field along the surface of thephotoconductive layer. These output pulses give some information on therelative one-dimensional distribution of light and dark areas of theimage, but provide no information about the specific light distributionof the light pattern. The pick-up device does not relate to derivingFourier transform representations of images. Another prior art device ofthis type is disclosed in U.S. Pat. No. 3,412,269 issued to Crittenden,Jr. in 1968. The patent discloses a transducer translatingelectromagnetic wave energy to ultrasonic wave energy. The deviceincludes a slab of cadmium sulfide which is exposed to light of aspecific wave length such that alternate dark and light bands areestablished along the acoustic propagation axis of the cadmium sulfide.The dark and light bands are regions of high and low electricalimpedance respectively. The disclosed device does not relate toconversions between optical images and Fourier series or transformrepresentations thereof. Still another prior art device of the type isdisclosed in U.S. Pat. No. 3,649,855 issued to Auld in 1972. Thedisclosed device relates to modulating the conversion of acoustical toelectrical energy by varying a light beam illuminating the. convertingmaterial. Again, the disclosed device does not relate to the conversionbetween optical images and Fourier transform representation thereof. Infact, applicants know of no prior art technique for directly obtainingelectrical sig nals which are spacial Fourier transform representationsof optical images.

SUMMARY OF THE INVENTION The invention relates to converting betweenpictorial information and electrical representations thereof, andrelates specifically to directly converting between pictorialinformation and Fourier transform representations thereof. It relies onthe discovery that in certain configurations of certain materials, thereare relationships between the electrical and mechanical properties of amaterial that allow deriving electrical signals representing pictorialinformation incident on the materials, and that applying such electricalsignals to certain configurations of certain materials results inreconstructing the original pictorial information.

Specifically, the invention reflects the discovery of a coupling betweencontrolled sound waves and optical images which allows obtainingelectrical signals that are functions of the spatial Fourier transformsof the optical images, and on the discovery of a coupling between light,controlled sound waves and electrical signals which are a function ofthe spatial Fourier transforms of optical images which allows directlyobtaining the optical images.

In accordance with the invention, such conversions between pictorialinformation and electrical signals representing the pictorialinformation are done directly, by devices which use no spatial scanning,operate at low illumination levels (with visible or infrared, coherentor incoherent light), and require neither high voltages nor highcurrents, such that the driving power for the sound waves may be of theorder of 1 watt. Such devices are inexpensive since they make use ofbulk or surface properties of materials such as common metals,semiconductors or dialectrics, and are rugged. The devices embodying theinvention produce electrical signals representative of the spatialFourier transform of pictorial information incident on the devices.Hence, these electrical signals can be used directly for sophisticatedpictorial information manipulation, which is not' picture by a constantamount, multiplying the frequencies by a constant magnifies the picture,and combining the last two properties yields an electronic zoomcapacity. Monitoring and correcting for rapid overall changes of phasein the signals generated by devices constructured in accordance with theinvention allows electronic image stabilization.

One specific example of a device in accordance with the inventioncomprises a medium which has an electrical property that varies as adefined function of pictorial information incident on it and as adefined function of periodic mechanical deformations of the medium. Themedium is subjected to a succession of different periodic mechanicaldeformations, andthe electrical property of interest is measured at suchdifferent periodic deformations to derive a succession of electricalsignals. These electrical signals serve as an electrical representationof the incident pictorial information. In particular, when themechanical deformations are caused by vibrating the medium at amultiplicity of different frequencies, each of the electrical signals isderived at a specific vibration frequency and represents the term forthat frequency of a Fourier series representation of the incidentpictorial information. When the mechanical deformation is vibration ofthe medium through a continuous frequency range, the resultingelectrical signal represents the Fourier transform (over finite bounds)of the incident pictorial information.

The fundamental principles of the invention can be illustrated by meansof a device which relies on the coupling between controlled sound wavesand an optical image to generate electrical signals which are functionsof the spatial Fourier transform of the image. In this specification,the term sound waves means phonon waves of any frequency, such as fromabout 10 Hz to megaor gigaHz, and is not limited to frequencies in theaudible range, and the term controlled sound waves? means sound waves inwhich the full wave vector is controlled in terms of magnitude anddirection. The term Fourier transform is used generically and includes,as will become evident below, special cases of the mathematical conceptof Fourier transforms, such as Fourier series or truncated Fouriertransforms. The term optical image is used to mean spatial variations inlight intensity, and the term one-dimensional image" is used to mean anoptical image in'which only the variations along one dimension are ofinterest.

The device which may illustrate the fundamental principles of theinvention comprises a fused quartz substrate and a transducer forgenerating a surface sound wave in the substrate. An intrinsicsemiconductor film, such as CdS, is deposited over a portion of thesubstrate, and a pair of metal contacts are placed over the film but areseparated from each other by a narrow strip of the film. An opticalimage is projected on the exposed strip of film, and a constant voltagedifference is established between the metal contacts across that narrowstrip of film. The transducer is then swept through a frequency range tovibrate the substrate, and hence the semiconductor film thereon at adiscrete or continuous succession of differentfrequencies. The

current across the film strip separating the metal contacts is measuredat different frequencies. Each measured current value is representativeof the term, for the particular frequency, of the Fourier transformrepresenting the incident optical image. A number of such narrow stripsof an intrinsic semiconductor film may be arranged next to each other toform a type of a two-dimensional photoconductive device whose resolutionin the direction transverses to the strips length is limited by thewidths of the strips.

A device for generating an electrical signal representation oftwo-dimension pictorial information comprises a configuration which issimilar to the onedimensional device, but includes means for generatinga controlled sound wave, which may be obtained for example by nonlinearcoupling of twojtransducers each operating at its own frequency, withthe result that sound wave beam steering may be accomplished byindependently varying the frequencies of the two transducers.Alternately, a steered sound wave beam may be obtained by using thenormal modes of an acoustical system. v

The conversion of pictorial information into an electricalrepresentation thereof may be accomplished altemately by utilizing bulkproperties of degenerate semiconductors and metals, e.g., by making useof strain perturbation of the photoconductivity of such materials. Forexample, a slightly p-type silicon bar which is vibrated at differentfrequencies can be utilized in accordance with the invention to generateelectrical signalswhich are Fourier transform representations of anoptical image incident on the bar.

There are uses of the devices described above which do not require thereconstruction of the pictorial information represented by theelectrical signals derived thereby. Pattern recognition and informationtransmission are two such uses. If it is required to recreate thepictorial information, two possible ways of doing so are to calculatethe inverse. transform of the electrical signals and to display it oncurrently available devices such as cathode ray tubes, or to utilize adirect solid state device constructed in accordance with the invention.

Pictorial information is reconstructed in accordance with the inventionthrough a coupling between light, sound waves and electrical signals.This can be illustrated by a device comprising two parallel polarizingplates having an angle of between their respective planes ofpolarization.

A plate of elasto-optical material (for example, KDP, Plexiglass, orLithium Niobate) is sandwiched between the polarizing plates. Theelasto-optical material has the property of locally changing the angleof polarization of the light passing through it when a mechanical strainis applied to it. The device further includes a transducer coupled withthe elasto-optical plate to produce strain patterns whose amplitude andphase are governed by electrical signals of the type of the signalsderived by the devices described above. Thus, each strain wave in theelasto-optical material plate forms a spatial Fourier component of thepictorial information which is to be reconstructed. The strain rotatesthe polarization locally, thus letting light through the secondpolarizing plate to a detecting device. The system acts like a lightvalve. To add the Fourier component output by this system, use is madeof devices such as an acoustic delay line which adds up and recirculatesthe individual Fourier component sound waves until the entire inversetransform (the acoustic image) is accumulated. Then an electronicshutter or flash tube can illuminate the sound wave complex and theresulting optical image can be focused on a screen.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic illustration'used in explaining fundamental principles of the invention.

FIGS. 2, 3 and 4 illustrate obtaining controlled sound waves in asubstrate.

FIG. 5 illustrates a device for obtaining a Fourier transformrepresentation of a one-dimensional optical image.

FIG. 6 illustrates a device similar to FIG. 5, but used to obtain alimited resolution second dimension of an optical image.

FIG. 7 illustrates a different type of a device for obtaining a Fouriertransform representation of a one dimensional optical image.

FIG. 8 illustrates a device for obtaining a Fourier transformrepresentation of a two-dimensional optical image. I

FIG. 9 illustrates another device for obtaining a Fourer transformrepresentation of a two-dimensional optical image.

FIG. 10 illustrates a device for obtaining an optical image from aFourier transform representation of the image.

FIG. 11 illustrated a different form of an output deriving means usefulin connection with the devices shown in FIGS. 5 through 9.

FIG. 12 illustrates the use of devices of the type shown in FIGS. 5through 9 for obtaining a Fourier transform representation of a colorimage.

DETAILED DESCRIPTION Before describing specific devices constructed andoperating according to the invention, it may be helpful to define someof the terminology used in this specification, and to consider certainfundamentals regarding image formation and the transformation of anoptical image into electrical signals which are Fourier transformrepresentations of the image.

The optical images discussed in this specification are formed byvariations in the intensity of light over a surface which is usually aplanar surface. For simplicity, only a two-dimensional planar surface,in which light intensity may vary both in the X and in the Y directionsis considered. A special case of such planar twodimensional opticalimages is a one-dimensional image, in which light varies only along asingle dimension, for example, only along the X direction. When Fouriertransforms are discussed in this specification, it should be clear thatit is not the pure mathematical concept of a Fourier transform, but atruncated Fourier transform, where the truncation is due to the factthat the quantities of interest are the Fourier transforms of imagesover finite areas. The mathematical concept of a Fourier transforminvolves an integral taken over indefinite bounds. However, since imagesof finite size are of interest to the subject invention, the integral isover finite bounds, and when the term Fourier transform is used, thismeans a transform which is truncated in some way. The term Fouriertransform also includes a plurality of Fourier series terms. When theterm sound wave is used in this specification, it means a wave of strainin a material, where the wave frequency can be any attainable frequency.It is emphasized that the adjective sound is not a limitation to audiblefrequencies; in fact frequencies of 10 Hz to megaor gigaHz are useful,and frequencies of kHz to 10 MHz have in face been used. The termcontrollable sound waves and steered sound waves mean sound waves inwhich the full wave vector (this means the magnitude and direction ofthe wave vector) can be changed selectively.

Some of the fundamental principles utilized in the subject invention maybe illustrated by referring to FIG. 1 which shows schematically aone-dimensional strip 1 made of a light sensitive medium. Suppose thatthe light incident on an incremental segment dX of the strip 1 producesa change dV, in the potential across the longitudinal ends of theincremental segment dx. Physical constraints require that the actualpotential difference which may be sensed be across a finite length.Suppose that this length has end points a and b. In that case, thepotential difference AV, sensed across the points a and b which definethe ends of the onedimensional photosensitive line would be da: da:

b an... L (X)dx (H) A standard approach to image sensing in the priorart is. to juxtapose many identical elements to form an array whichallows the separate sensing of each element. The subject invention movesaway from this approach, and utilizes a single device which generateselectrical signals representing the entire image incident on the device.To explain a fundamental relationship of the subject invention, referagain to the hypothetic onedimensional photosensitive strip discussedabove, and suppose that the change in voltage across the longitudinalends of the incremental segment dx is also dependent on a harmonicdisturbance of the segment, which harmonic disturbance is in the formwhere k is the propagation constant, w is the temporal frequency, and iis a proportionality constant. Now the voltage drop across thehypothetical strip defined by the end points'a and b becomes The firstterm of the right-hand side in the above expression is simply thepotential difference across the segment defined by the end points a andb in the absence of a harmonic disturbance. The second term in the aboveexpression is the term of interest because it takes the form of theFourier integral of the photon flux representing the optical imageincident on the hypothetical one-dimensional strip. This second term issmall, but is detectable, because it is time-varying, while the firstterm is not. Calling the second term of the above expression AV resultsin the following expressions:

rb l iwt iKx i(wt0) r i 0.2.6 v J; (X)e da: |AV|e (H) where both themagnitude lA Vl and phase 0 are dependent on the light intensity (X) andon the propagation constant k. The propagation constant k=k(w) is thedispersion relation for sound waves in the material of the strip. It isseen thus that the voltage AV has the form of the component of theFourier transform of the light intensity distribution (X), where (X) 0outsidethe end points a and b, at the spatial frequency. Thecorrespondence is then and [V vi febi esefit fhiiy iii li ghtint ensitydistribu tion (X) along the hypothetical line between the end points aand b. The propagation constant k is a function of the frequency.

A fundamentally important aspect of the above discussion is that sinceeach Fourier component contains information about the entireone-dimensional strip defined by the end points a and b, the resolutioncapabilities of a detector based on the above discussion is notdetermined by the distance between the end points, but is limited by thehighest spatial frequencies which may be obtainable, and, in anyspecific device, by frequency response limitations of the material andof the sensing electronics. A further fundamentally important aspect isthat a single device, which is many times larger than its resolution,can be used to sense an entire image.

The above discussion indicates why the availability of controlled soundwaves is essential for practicing the subject invention. Since all formsof devices operating according to the invention use some form ofcontrolled sound waves, it may be appropriate to discuss how such wavescan be obtained.

In the case of one-dimensional optical images, controlling the frequencyof vibration of a medium is sufficient. This can be done by vibrating amedium, such as a fused quartz substrate by means of any one of a numberof conventional transducers. For example, referring to FIG. 2, a bar 2of a suitable material, such as fused quartz is vibrated by means of aconventional transducer 3 coupled to the bar 2 by means of a wedge 4 anddriven by a suitable frequency generator 5. Depending on the frequencyof the electrical signal from the source 5 and depending on the relativedimensions, materials and orientation of the bar 2 and transducer 3, asurface sound wave or a bulk sound wave of a particular frequency isinduced in the bar 2. The vibration of the bar 2- may be in a standingwave mode, or alternately the bar.2 may be terminated in an acousticalabsorber 6. The source 5 may be a sweep frequency generator which can beswept, for example, between the frequencies of kHz to 10 MHz to drivethe transducer 3. However, the transducer 3 produces an output onlywhenever an odd multiple of half wavelength is equal the length of thetransducer. Thus, the transducer 3 produces only odd harmonics. Twotransducers of this type may be used, one half as long as the other, togenerate a series of harmonics in which every fourth harmonic ismissing. Using three transducers of the same type, each succeeding beingone-half 'as long as the preceding one, allows the generation of aseries of harmonics in which every eightharmonic is missing, etc.Altemately, thin film transducers may be used which offer better controlof the output characteristics. It is noted that several harmonics of afundamental frequency may be present in the bar 2 at any one time.

A more complex control of a sound wave is needed in the case oftwo-dimensional optical images. If the str ain in a medium is a functionof e wand m. q X q Y, both q, and q,, must be selectively changeable toobtain a sound wave that can be controlled in the required manner.

One way to obtain the required two-dimensional control of sound wavesutilizes the fact that many materials, both crystalline andpolycrystalline, are susceptible to strain amplitudes which result insignificant mixing of the waves due to force terms proportional to thesquare of the strain. If the strain frequencies are less than 100 MHz, aclassical picture of the acoustic waves is adequate.

If two transducers 3a and 3b (see FIG. 3) driven by sources 5a and 5brespectively, are used to generate two sound waves in the same medium 2,namely,

21 ACOS E Bcos (w r q y) (e-8) and these waves are mixed, the result ofnonlinear coupling of the two waves is a wave which has four componentsthat propagate in the original directions established by the transducers3a and 3b, and a fifth term which can be expanded to yeild C5/2 cos [(W+W t (q Xq,,Y)] (5-9) where Vfiqx w 2 and Vfqv W 2. Thus the sum anddifference frequency terms are steerable and a controlled sound wave canbe so obtained. Both q, and q can be varied at will by changing thefrequencies at which the two transducers 3a and 3b are driven by thesources 5a and 5b. The multiplier C involves the third order elasticconstant and the product of the original strains AB. Note that thedifference term contains the (-fiF) term. The sum term contains theconjugate wave vector and should steer away from a region 7 in FIG. 3,which can be used as an image detecting region.

The first four terms of the mixed wave should also propagate away fromthe image detecting region 7, leaving only the difference term to coupleto a light image that may be projected on the region 7.

W C (II/d) (e-lO) h es ter wrainthisasie ate r. W e with displacement1'),- in the z direction only, and propagation in the x and ydirections. The equation of motion for the displacement of such a shearwave is V C28 V2 i (c-m where C is the shear velocity of sound in thematerial of the substrate 2 in FIG. 4, and i is the region index. Thedisplacement in the region 1 shown in FIG. 4 can be expressed asfollows:

By substituting the expression (e-12) into the expression (-1 l thefollowing disperson relation can be obtained for region 1 W /C (Zmrr/a)(2n'Ir/b) (el3) By using similar expressions for the other regions shownin FIG. 4 and appropriate boundary conditions, it can be shown thatB=D=F=O. Thus, a set of normal modes are obtained which can be selectedby driving the regions 2 or 3 in FIG. 4 by identical transducers at thesame frequency. In order to insure that no degeneracy occurs in the lastexpression, the ratio a/b is chosen to be irrational, for example, b(1r/2) a. The constraint w C (1r/2) can be satisfied for frequenciesless than 100 MHz by choosing, for example, d 0.188 mm, which is areasonable thickness of the substrate 2 in FIG. 4. If the dimension ashown in FIG. 4 is approximately 35 mm, then the expression (e-l3) canbe solved, for a particular material, to yield f= 107600 (e-l4) for allm,n integers.

An exemplary device embodying the subject invention, as applied to thecase of one-dimensional optical images, is shown in FIG. 5. The deviceemploys sound wave modulation of the photoconductance of an intrinsicsemiconductor, and generates electrical signals which are a Fouriertransform representation of a onedimensional optical image incident on adetecting strip.

Referring to FIG. 5, a substrate 10 which may be a fused quartz bar, hasdeposited on its top surface a film 12 of a photoconductive intrinsicsemi-conductor such as CdS. The film 12 is flanked and is partlyoverlapped by two metal contacts 14 and 16 which may be aluminum filmstrips and which are in electrical contact with the semi-conductor film14. The electrical contacts 14 and 16 are spaced from each other by asmall distance to expose a thin strip of the semi-conductor film 12 to alight pattern projected from above the substrate 10 by a projector 18.

In one exemplary device, the exposed photodetecting strip of thesemiconductor film 12 is approximately 0.006 inches wide and isapproximately 15mm long, and the semiconductor film 12 is approximately5,000 A thick. The semiconductor film in the exemplary device ispolycrystalline Cds which, however, tends to have the C axis of theindividual crystallites aligned perpendicular to the plane of the film12. The film l2 exhibits no net piezoelectric effect in the plane of thefilm. A transducer 20 is acoustically coupled with the top surface ofthe substrate 10 through a Plexiglass wedge 22 and is driven by a sweepfrequency generator 24. This arrangement allows the generation of asurface acoustical wave which propagates along the top surface of thesubstrate 10 from left to right in FIG. 1, i.e., from the transducertowards and through the region under the semiconductor film 12. Theright-hand end of the substrate 10, i.e., the end which islongitudinally opposite the end on which the transducer 20 is mounted iswrapped in acoustical absorbing tape 26 which is for the purpose ofabsorbing substantially without reflection surface sound wavespropagating toward the tape 26 from the transducer 20.

A constant potential difference is established across the electricalcontacts 14 and 16 by means of a constant voltage source 28, and anyvariations in the conductance of the semiconductor 12 are measured bymeasuring the current through a resistor 30 by means of an AC isolatedpreamplifier 32 feeding a phase synchronous detector 34 which alsoreceives as an input an output from the sweep frequency generator 24that carries information about the instantaneous frequency of thegenerator 24. The detector 34 records an electrical signal thatrepresents the instantaneous magnitude and phase of the current acrossthe contacts 14 and 16.

In operation of the device illustrated in FIG. 5, the sweep frequencygenerator 24 is swept from, for example, kHz to 10 mI-Iz, to drive thetransducer 20 to generate surface sound waves as discussed in connectionwith FIG. 2. The conductance of the exposed strip of the semiconductorfilm 12, i.e., the strip which is between the contacts 14 and 16, ismodulated both by the light pattern projected on it by means of thelight pattern projector 18 and by the frequency of the surface wavegenerated by the transducer 20. At each sound wave frequency, theconductance measured across the electrical contacts 14 and 16 isrepresentative of the term at that frequency of the ourier seriesrepresenting the entire one-dimensional light pattern from the projector18.

1 1 As a qualitative mathematical discussion of the device of FIG. 5,assume that the conductance per unit length of the exposed strip of thesemiconductor film 12 can be expressed as follows:

AG 81) +81. 4 8m Bts (X) 2 (el) where g is the dard conductance in theabsence of strain, g is the change in the conductance with light where(b is the photon flux in watts/m g is the change of dark conductancewith strain where Z E e is the strain due to the surface wave generatedon the top surface of the substrate 10 by the transducer 20, and q Y) Eis the change in the conductance with light and strain.

The current AI per unit length of the exposed strip is then AI AGV whereV, is the constant voltage applied across the electrical contacts 14 and16 by the constant voltage source 28. The total current I measured bythe preamplifier 32 and the detector 34 across the resistor 30 is then b1= V. L AGdrc (Gila) where X is along the length of the expoled stripwhose longitudinal end point are a and b. The AC component i of thiscurrent is However, for the specific material used in messes; plarydevice in FIG. 1, namely, CdS, the dark tolight conductance ratio g /gis of the order of 3 00 to 1. Thus, for normal light intensities, the ACcomponent of the current is approximately b I filffiitt fq tFfi f t Theexpression (e-l8) above indicates clearly that the current which ismeasured by the preamplifier 32 and the detector 34 is approximatelyproportional and corresponds to the Fourier transform of the lightintensity pattern projected by means of the project or 18.

In the experimental configuration shown in FIG. 5, a specific devicewhich has been tested has approximately 100 K0 light resistance andgenerates voltage signals between SOuv-and 2mV AC across a IOKQ resistor30. Since it is desirable to operate with lower impedances than 100K!)several devices of the type shown in FIG. 5 may be connected inparallel, with the result being a device of the type shown in FIG. 6.

manner electrical signals which are substantially the term coefficientsof a Fourier series representation of the light pattern, and aretherefore a Fourier transform representation of the light pattern.

A device corresponding generally to the surface effect device shown inFIG. 5, but utilizing bulk properties of certain materials, such asdegenerate semiconductors and metals, is illustrated in FIG. 7. Thedevice shown in FIG. 7 makes use of strain perturbation of thephotoconductivity of a material, and comprises a bar 36 of a materialsuch as a slightly p-type silicon, a light pattern generator 38 forprojecting on the bar 36 a light image which varies only in thedimension along the length of the bar 36, means for vibrating the bar36, means for causing a constant current flow through the bar 36 andmeans for detecting the potential difference across the longitudinalends of the bar 36.

The means for vibrating the bar 36 at selected different frequenciescomprise a sweep frequency generator 40 which is capable of generatingfrequencies from the audio range up to about 100 MHz and which drives atransducer 42 which is acoustically coupled to an acoustic transformerglass pyramid 44 that is in turn acoustically coupled to onelongitudinal end of the bar 36. The opposite longitudinal end of the bar36 is acoustically coupled to a mass of an acoustic absorbing material46 which absorbs substantially without reflection waves propagating fromthe transducer 42 toward the absorbing material 46. A constant currentsource 48 is suitably connected to the opposite longitudinal ends of thebar 36 to establish a constant current flow through the bar 36, and apreamplifier 50 is suitably connected to the longitudinal ends of thebar 36 to measure the instantaneous potential difference therebetween.The output of the preamplifier 50 is connected to a phase synchronousdetector and frequency compensator network 52 which receives as anotherinput an output from the sweep frequency generator 40 carryinginformation identifying the instantaneous frequency of the generator 40.The purpose of the unit 52 becomes apparent from the discussion givenbelow of the mode of operation of the device shown in FIG. 7. The sweepfrequency generator 40, the transducer 42 and their electricalconnections are electrically shielded from the rest of the device shownin FIG. 7 by means of an electrical shielding membrane 54.

In operation, the sweep frequency generator 40 is swept through asuitable range of frequencies to cause thereby bulk wave vibration ofthe bar 36 at a succession of different frequencies defined by thecorresponding harmonics of the transducer 42. At each of thesefrequencies, the potential difference across the longitudinal ends ofthe bar 36 is detected by means of the units 50 and 52, which alsodetect the phase of that voltage signal relative to, fo example, thesignal from the generator 40. At each frequency, the voltage detectedacross the ends of the bar 36 corresponds to the term for that frequencyof a Fourier series representation of the one-dimensional light patternprojected on the bar 36 by the light pattern generator 38.

Inreality, one does not obtain the Fourier transform of the lightpattern, but rather the transform of the light generated charge carrierdistribution within the measured portion of the bar 36. Thisdistribution is not identical to the light intensity pattern since thecharge carriers tend to defuse away from the place where they aregenerated. This results in a smearing of the light pattern which causesan attenuation of the high frequency Fourier components. This can becorrected to a certain extent by a frequency compensating network whichis included in the phase synchronous detector and frequency compensatornetwork 52. The frequency compensator network simply amplifies the inputsignals which correspond to higher vibrational frequencies. In aspecific experimental device, the bar 36 may be 3,500 ohm-cm slightlyp-type silicon having a charge carrier lifetime of about 3ms in adiffusion length of about 5mm. This diffusion length would limitresolution to a few millimeters.

Two-dimensional devices for generating Fourier transform representationsof two-dimensional pictorial information are both more important andmore complicated. Two-dimensional devices constructed and operating inaccordance with the invention may employ essentially the same principlesas the one-dimensional bulk effect and surface effect devices. Thesubstantive difference between the one and two-dimensional devices isthat the two-dimensional devices require a steerable controlled soundwave, so that the second dimension of the pictorial information can beobtained.

One exemplary two-dimensional device utilizes photomissivity and isshown schematically in FIG. 8. The device in FIG. 8 comprises asubstrate 56 of a material such as a fused quartz plate which hasdeposited on one of its large faces a photocathode film 58. Aphotoelectron collector plate 60 is positioned parallel to thephotocathode film 58 and is spaced therefrom by a suitable smalldistance. A constant potential difference is established between thefilm 58 and the collector 60 by means of a constant voltage source 62,and the emission current between the film 58 and the collector 60 ismeasured by means of measuring the potential difference across aresistor 64 by a preamplifier 66 feeding a phase synchronous detector 68which also receives a frequency input from transducer 72. A lightpattern is projected through the substrate 56 onto the photocathode film58 by means of a projector 70. The substrate 56 is vibrated in suitablemodes and at suitable frequencies by means of transducers 72 which aresuitably coupled acoustically to the substrate 56. The device operatesin vacuum.

As a possible qualitative mathematical description of the operation ofthe arrangement shown in FIG. 8, consider the following. Underillumination electrons are emitted from a metal plate in vacuum inaccordance with the Einstein equation mv =fi w (ev) (ti-l9) wherev,,,,,, is the maximum speed of emitted electrons, w,, is the frequencyof light, V is the applied accelerating potential, and 5 is the surfacework function. The current density magnitude is J env 23p (7) V (e-ZO)where n is the emitted electron density, p(T) is the photon densityabsorbed, and B is the quantum efficiency of the process. Assuming thatthe temperature is low and that the operation is reasonably close tocut-off, most electrons are emitted with the maximum speed v,,,,,,,..The current densit ma nitude then becomes J efipfi) 2/m (hwfi -Fevl(e-2l) The change in the work function with strain is due to the changeof the Fermi energy when strained since the vacuum level is fixed. Thusapt/8255 (8-25) where p. is the Fermi energy. The value of this changecan be calculated and it can be shown that this perturbation of d), issmall, but measurable. If the system is arranged so that hwp eV (e-26)where F (l/,,) ([8/8]E E 'y,,e w- The following expression can becalculated for the total current collected:

f ar -n1 where the integral is over the area of the photoemitter.

From the above expression, it is seen that the component of the currentwhich varies at the strain frequency w is proportional to the qcomponent of the Fourier transform of the light intensity. Detecting theAC component and scanning in w provides the entire transform. Byincluding a photomultiplier, the parameter B can be manipulated toobtain the required sensitivity.

The principles discussed above are applicable to the arrangement shownin FIG. 8, where electrons are emitted from the photocathode film 58under the effect of the light pattern projected from the projector andunder the effect of the strain induced by means of the transducers 70,and the current is the current through the resistor 64 as measured bythe detector 68.

It should be apparent from the qualitative mathematical discussion abovethat the derivation of the output signal, namely, the current throughthe resistor 64 in FIG. 8, involves integrals whose intergrands containproducts of the intensity distribution of the image projected onto thephotocathode film 58 and the strain deformation of the substrate 56. Ifthe strain is proportional to e" 'u the detected current signal shouldbe proportional to the twodimensional Fourier transform, where if q, ,9.This condition requires steering of the acoustical beam causingdeformation of the substrate 56. In particular, q and q, must bechangeable essentially by varying the frequencies of theelectricalsignals that may be used to drive the transducers 72, asdiscussed earlier in connection with FIGS. 3 and 4.

When the device illustrated in FIG. 8 is used for converting pictorialinformation into an electrical signal representation thereof, it is notnecessary to geometrically separate the various components of the strainwave, since the difference frequency can be detected in a simple mannerand the other signal components can be disregarded. However, in devicesfor converting from electrical signal representations of pictorialinformation into pictorial information, no frequency discrimination ispossible, and any sound wave in the device will modulate the outputpictorial information. It is therefore necessary to find means by whichit would be possible to geometrically separate the various components ofthe strain wave in the substrate. One possible method is illustrated inFIG. 9, and comprises a substrate 56 similar to the substrate 56 in FIG.8 and transducers 74 and 76 which are acoustically coupled to thesubstrate 56 and are driven respectively by sweep frequency generators74a and 76a whose frequencies are independently variable. The strainsinduced in the substrate 56 by means of the transducers 74 and 76 couplenonlinearly (in the manner discussed in connection with FIG. 3) in aregion 56a and couple linearly in a region 56b. The unmixed strain wavesinduced by the transducers 74 and 75 propagate away from a photocathodefilm area 78 which is in the linear mixing region 56b, while thedifference strain wave passes through the photocathode film area 78. Thespecial cases (q,,O) and (O,q,,) can be obtained by inducing a strainwave in the substrate 56 by means of one of the transducers 80 and 82which are appropriately labelled.

A device equivalent to the two-dimensional device shown in FIG. 8, butrelying on the strain dependence of photoconductance and utilizing thebasic principles discussed in connection with the device of FIG. 5requires a steerable acoustic wave and must take into account the factthat the resistance of a rectangular plate is not proportional to itsarea, that is the product of its dimension, but is proportional ratherto the ratio of its dimension. A one-dimensional device does not presentthis difficulty since we allow no variation in the direction transverseto the current. A two-dimensional device, however, must have provisionsfor taking'into acv count this fact.

Pictorial information which has been converted to the electricalrepresentation discussed above can be recreated either by calculatingthe spatial pattern (inverse transform) and displaying it on-currentlyavailable devices such as cathode ray tubes, or by using a direct solidstate projector constructed and operating in accordance with anotheraspect of the invention.

A pictorial information reconstruction system is illustrated in FIG. andcomprises polarizing plates 86 and 90 and a plate 88 of photoelasticmaterial which is located intermediate the polarizing plates 86 and 90.The planes of polarization of the plates 86 and 90 are at a 90. angle toeach other. For example, the polarization plane of the plate 86 is inthe horizontal direction while the polarization plane of the plate 90 isin the vertical direction. The intermediate plate 88 is of aphotoelastic material, such as for example, KDP or Plexiglass and hasthe property of locally changingthe angle of polarization of lightpassing through it when a mechanical strain is applied to it. Thisproperty of elasto-optical materials is well recognized; for example,KDP is used as a light modulator in quantum electronics and Plexiglassis used to demonstrate stress patterns in various mechanical components.Electro-mechanical transducers 92 and 94 are acoustically coupled withthe plate 88 to produce strain patterns similar to the strain patternsdiscussed in connection with FIGS. 3 and 4. The amplitude and phase ofthe strain patterns are governed by electrical signals supplied bysignal generators 92a and 94a whose outputs are in turn governed byelectrical signals of the type derived by the detector 68 in FIG. 8.Thus, each strain wave in the elasto-optical material plate 88 forms aspatial Fourier component of the pictorial information which is to bereconstructed bythe arrangement shown in FIG. 10. This strain rotatesthe polarization locally, thus letting light through the polar izationplate which light comes from a collimated light source to the left (inFIG. 10) of the polarization plate 86. The light which is transmittedthrough the polarization plate 90 can be applied, as a signal, to anacoustic delay line which adds up and recirculates the individualFourier component sound waves applied to it until the entire inversetransform (the acoustic image) of a particular optical image isaccumulated. Then an electronic shutter or flash tube can illuminate thesound wave complex and the optical image can focus on a screen. Theplate 88 may be of materials such as lithium niobate.

In connection with the devices illustrated in FIGS. 5 through 9, itshould be noted that in many cases the substrate material vibrates at afundamental frequency and simultaneously vibrates at one or moreharmonics of that fundamental frequency. This can be utilizedadvantageously in simultaneously deriving electrical signalsrepresentative of the Fourier transforms for these several simultaneousfrequencies. Thus, referring to FIG. 11, the block labelled devicerepresents for example the device shown in FIG. 5 and is vibrated bymeans of a source 102. An optical projector 104 projects an opticalimage on the device 100. At each frequency of vibration of the device100, the output signal derived from the device 100 is a Fouriertransform representation of the optical image projected on the device100 by the optical projector 104. However, since the device 100 in factvibrates at a fundamental frequency and one or more detectable harmonicsof that fundamental frequency, it is advantageous to simultaneouslydetect the output at each of these several different frequencies ofvibration of the device 100. Accordingly, the output of the device 100is directed simultaneously to a number of frequency filters, forexample, frequency filters l06a, 106b, and 106c. Each of the frequencyfilters passes only a frequency band corresponding to the fundamentalfrequency or to one of the detectable harmonics of the fundamentalfrequency of vibration of the device 100. Thus, each of the frequencyfilters provides an output which is an electrical signal representativeof the Fourier transform corresponding to the frequency within the bandpass of the filter. These output signals of the filters are recorded ata recorder 108. The filters 106a, 106b, and l06c are controlled by afilter control 110 which receives an input from the source 102 andprovides outputs which determine the band pass frequency of the filters.For example, the filter 106a may be set by means of the filter control110 to pass only electrical signals which correspond to theinstantaneous fundamental frequency of vibration of the device 100, thefilter 10Gb may be set to pass only electrical signals which correspondto a given harmonic of that instantaneous fundamental frequency ofvibration, and the filter 106s may be set to pass only electricalsignals which correspond to another harmonic of the instantaneousfundamental frequency of vibration of the device 100.

So far, an optical image was considered as represented by the spatialdistribution of light intensity forming the image. However, the devicesdiscussed above may be utilized in a system for obtaining a Fouriertransform representation of a color image, by means of a system of thetype illustrated in FIG. 12. In FIG. 12, an optical image produced by aprojector of the type discussed in connection with FIG. is projectedtoward a device 112, such as a common prism, for separating the opticalimage into the three primary additive colors, blue, green and red. Theresult is three separate optical images, each in only one of the primaryadditive colors. A device of the type shown in FIGS. 5 through 9, forexample a device of the type shown in FIG. 5, is provided for each ofthese three colors. For example, a device 114a is provided for obtainingthe Fourier transform representation of the blue portion of the image, adevice 11% is provided for obtaining the Fourier transformrepresentation of the green portion of the image, and a device 1140 isprovided for obtaining the Fourier transform representation of the redimage. Corresponding recording devices 116a, 116k and 116s are providedto record the electrical signals output by the devices 114a, ll-4b and1146 respectively.

We claim:

1. A device for generating an electrical signal representation of aone-dimensional optical image comprismg:

a substrate of a material capable of vibrating and means for vibratingthe substrate at a plurality of different frequencies;

. a film of a material whose electrical conductance varies as a functionof an optical image incident thereon and as a function of time and spacevarying strain waves therein, said film secured to a surface of thesubstrate to vibrate therewith;

a pair of electrically conductive contacts disposed over a portion ofthe film in electrical contact therewith and spaced from each other toexpose a narrow strip of film;

means for projecting an optical image on the exposed film strip;

a constant voltage source connected to the two contacts to establish apotential difference therebetween across the exposed film strip; and

means for detecting the current between the two contacts across the filmstrip at a plurality of different frequencies of vibration of thesubstrate to derive a plurality of electrical signals each being arepresentation of at least one dimension of the entire optical image ata defined frequency of vibration of the substrate.

2. A device as in claim 1 wherein each of said electrical signals is acomponent of a Fourier Transform representation of the optical image atthe frequency of vibration of the substrate at which the signal isderived.

3. A device as in claim 2 wherein each of said electrical signalsrepresents a selected component of a Fourier Transform representation ofthe light intensity distribution along at least one dimension of theentire optical image incident on a defined portion of the film for thefrequency of vibration of the substrate at which the electrical signalis derived.

4. A device as in claim 2 wherein each of said electrical signals is aselected component of a Fourier Transform representation of the lightintensity distribution along a single dimension of the entire opticalimage incident on a defined portion of the film for the frequency ofvibration of the substrate at which the electrical signal is derived.

5. A device as in claim 1 wherein the means for vibrating the substratecauses the propagation of surface waves in the substrate.

6. A device as in claim 1 wherein the film comprises substantially aphotoconductive intrinsic semiconducfor.

7. A device as in claim 1 wherein the substrate comprises substantiallyfused quartz and the film comprises substantially polycrystallinecadmium sulfide.

8. A device for generating an electrical signal representation of anoptical image comprising:

a medium comprising a film capable of undergoing time and space varyingstrain disturbances and having an electrical property which varies as afunction of an optical image incident on the film and as a function oftime and space varying strain disturbances in the film;

means for causing the incidence of an optical image on the film;

means for causing a plurality of different time and space varying straindisturbances in the film; and

means for measuring said electrical property of the film at a pluralityof said different strain disturbances therein to derive a plurality ofelectrical signals representing said optical image.

9. A device as in claim 8 wherein the medium comprises a substrateacoustically coupled with the film and capable of undergoing time andspace varying strain disturbances in acoustic coupling with the film.

10. A device as in claim 9 wherein each of said electrical signals is afunction of the light intensity distribution along at least onedimension of the entire optical image incident on a defined portion ofthe film and on the strain disturbances in the film at which the signalis derived.

11. A device as in claim 9 wherein said optical image incident on thefilm is two-dimensional and wherein each of said electrical signals is arepresentation of the entire two-dimensional optical image incident on aselected portion of the film for the strain disturbances at which thesignal is derived.

12. A device as in claima 9 wherein said electrical signals are selectedcomponents of a Fourier Transform representation of the optical image.

13. A device as in claim 9 wherein each of said electrical signals is aselected component of a Fourier Transform representation of the lightdistribution along at least one selected dimension of the optical image.

14. A device as in claim 9 wherein each of the electrical signals is aselected component of a Fourier Transform representation of the lightdistribution along a single dimension of the optical image.

15. A device as in claim 9 wherein each of the electrical signals is aselected component of a Fourier Transfonn representation of the lightdistribution along two noncongruent dimensions of the optical imageincident on a selected portion of the film.

16. A device as in claim 8 wherein the means for causing a plurality ofdifferent strain disturbances in the film comprises means for causingthe propagation of a surface strain wave in the film.

17. A device as in claim 16 wherein the means for causing straindisturbances comprises means for vibrating the film with a controlledstrain wave wherein the wave vector of the strain wave is controlled inat least two noncongruent dimensions.

18. A device as in claim 17 wherein each of said electrical signals is acomponent of a Fourier Transform representation of the entiretwo-dimensional optical image incident on at least a portion of thefilm.

19. A device as in claim 16 wherein each of the electrical signals is arepresentation of at least a component of the electrical conductivity ofthe film.

20. Method of obtaining an electrical signal representation of anoptical image comprising:

causing the incidence of an optical image on a medium having anelectrical property which varies as a function of an optical imageincident on it and as" a function of time and space varying surfacestrain wave disturbances in the medium;

causing a plurality of different time and space varying surface strainwave disturances in the medium; and

measuring said electrical property of the medium at a plurality of saiddifferent surface strain wave disturbances in the medium to derive aplurality of electrical signals representing said optical image. 21. Amethod as in claim 20 wherein each of said electrical signals is arepresentation of the light distribution along at least one dimension ofthe optical image incident on at least a portion of the medium.

22. A method as in claim 20 wherein each of said electrical signals is arepresentation of the light distribution along two noncongruentdimensions of the optical image incident on at least a portion of themedium.

23. A method as in claim 20, wherein each of said electrical signals isa selected component of a Fourier Transform representation of saidoptical image incident on at least a portion of the medium.

24. A device as in claim 20 wherein each of said electrical signals is aselected component of a Fourier Transform representation of the lightdistribution along at least one dimension of the optical image incidenton a selected portion of the medium.

25. A method as in claim 20 wherein each of said electrical signals is aselected component of a Fourier Transform representation of thetwo-dimensional light distribution of the optical image incident on aselected I portion of the medium.

26. A method as in claim 20 including controlling the wave vector of thestrain wave in the medium in at least the two dimensions of the imageincident on the medium.

27. A method as in claim 26 wherein each of said electrical signals is aselected component of a Fourier Transform representation of the entiretwodimensional optical image incident on a selected portion of themedium.

28. A method as in claim 20 wherein said electrical property of themedium is at least a selected component of the electrical conductivityof the medium.

29. A device for generating an electrical signal representation of anoptical image comprising:

a substrate of a material capable of undergoing time and space varyingstrain disturbances and a film secured to a surface of the substrate toundergo strain disturbances therewith, said film having an electricalproperty which varies as a function of an optical image incident on thefilm and as a function of time and space varying strain disturbances inthe film;

means for causing the incidence of an optical image on at least aportion of the film;

means for causing a plurality of different time and space varying straindisturbances in the film; and

means for measuring said electrical property of the film at a pluralityof said different strain disturbances in the film to derive a pluralityof electrical signals representing said optical image, said measuringmeans comprising an interdigitated plurality of first and secondelectrically conductive contacts disposed over portions of the film inelectrical contact therewith, with the first contacts electricallyconnected to each other and with the second contacts electricallyconnected to each other, but with the first and second contacts spacedfrom each other along the film.

30. A device as in claim 29 wherein the means for causing straindisturbances causes the propagation of a surface strain wave along thesurface of the substrate facing the film.

31. A device as in claim 29 wherein each of said electrical signals is aselected component of a Fourier Transform representation of the opticalimage incident on at least a portion of the film.

32. A device as in claim 29 wherein the substance is substantially fusedquartz and the film is substantially a photoconductive intrinsicsemiconductor.

33. A device for obtaining an electrical signal representation of anoptical image comprising:

a substrate of a material capable of undergoing time and space varyingstrain disturbances and a photocathode film secured to a surface of thesubstrate to undergo said strain disturbances therewith;

a photoelectron collector plate facing the photocathode film but spacedtherefrom to. collect electron current emitted from the photocathodefilm and means for establishing a potential difference between thephotocathode film and the photoelectron collector plate;

means for causing the incidence of an optical image on the photocathodefilm;

means for causing a plurality of different time and space varying straindisturbances in the substrate and in the photocathode film; and

means for measuring an electrical parameter of the current between thephotocathode film and the photoelectron collector plate'at a pluralityof said different strain disturbances in the photocathode film to derivea plurality of electrical signals representing the optical image.

34. A device as in claim 33 wherein each of said electrical signals is aselected component of a Fourier Transform representation of the lightdistribution along at least one dimension of the optical image.-

35. A device as in claim 33 wherein each of a selected plurality of saidelectrical signals is a selected component of a Fourier Transformrepresentation of the entire two-dimensional optical image incident on aselected portion of the photocathode film.

means for propagating through said intermediate plate strain patternswhose amplitudes and phases are governed by said electrical signals;

a light source for illuminating one side of the combination of the threeplates; and

means for displaying the light pattern emerging from the side of thecombination of the plate which is opposite the light source.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,836,712 Dated September 17, 1974 Inventor(s) Philipp G. Kornreich andStephen T. Kowel It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Title page, before "ABSTRACT" the following should be inserted:Attorneys: Cooper, Dunham, Clark, Griffin & Moran-- Col. 5, line 43,"Fourer" should read Fourier line 48, "illustrated" should readillustrates Col. 7, line 7, "i" should read I Col. 9, line 23, theequation should read Col. 9, line 28, equation (e-l2) should read z 2mTr2n V Q [Acos xcos OS a X Sln fig-Y +Dsin xcos 2 y Fsin zmw xsin y] eCol. 10, line 63, "ourier" should read Fourier Col. 11, line 8, "dard"should read dark line 26, "expoled" should read exposed Col. 14, line61, "twodimensional" should read two-dimensional Col. 18, line 52,"claima" should read claim Col. 20, line 32, "substance" should readsubstrate Signed and sealed this 18th day of March 1975.

SEAL) Attest:

C. MARSHALL DANA RUTH C. I'ZASON Commissioner of Patents ArrestingOfficer and Trademarks UNITED STATES PATENT OFFICE CERTIFICATE OFCORRECTION Patent No. 3,836,712 Dated September 17, 1974 Inventor(s)Philipp G. Kornreich and Stephen T. Kowel It is certified that errorappears in the above-identified patent and that said Letters Patent arehereby corrected as shown below:

Title page, before "ABSTRACT" the following should be inserted:Attorneys: Cooper, Dunham, Clark, Griffin & Moran-- Col. 5, line 43,"Fourer" should read Fourier line 48, "illustrated" should readillustrates Col. 7, line 7, "i" should read Col. 9, line 23, theequation should read Col. 9, line 28, equation "(e-l2)" should read 22mTr 2n v Q [Acos xcos -g -y Bcos a X sln g-y +Dsin xcos y Fsin zmTTxsin y] e a a b Col. 10, line 63, "ourier" should read Fourier Col. 11,line 8, "dard" should read dark line 26, "expoled" should read exposedCol. 14, line 61, "twodimensional" should read two-dimensional Col. 18,line 52, "claima" should read claim Col. 20, line 32, "substance" shouldread substrate Signed and sealed this 18th day of March 1975.

(SEAL) Attest:

C. MARSHALL DANE? RUTH C. H4301! Commissioner of Patents AttestingOfficer and Trademarks

1. A device for generating an electrical signal representation of aone-dimensional optical image comprising: a substrate of a materialcapable of vibrating and means for vibrating the substrate at aplurality of different frequencies; a film of a material whoseelectrical conductance varies as a function of an optical image incidentthereon and as a function of time and space varying strain wavestherein, said film secured to a surface of the substrate to vibratetherewith; a pair of electrically conductive contacts disposed over aportion of the film in electrical contact therewith and spaced from eachother to expose a narrow strip of film; means for projecting an opticalimage on the exposed film strip; a constant voltage source connected tothe two contacts to establish a potential difference therebetween acrossthe exposed film strip; and means for detecting the current between thetwo contacts across the film strip at a plurality of differentfrequencies of vibration of the substrate to derive a plurality ofelectrical signals each being a representation of at least one dimensionof the entire optical image at a defined frequency of vibration of thesubstrate.
 2. A device as in claim 1 wherein each of said electricalsignals is a component of a Fourier Transform representation of theoptical image at the frequency of vibration of the substrate at whichthe signal is derived.
 3. A device as in claim 2 wherein each of saidelectrical signals represents a selected component of a FourierTransform representation of the light intensity distribution along atleast one dimension of the entire optical image incident on a definedportion of the film for the frequency of vibration of the substrate atwhich the electrical signal is derived.
 4. A device as in claim 2wherein each of said electrical signals is a selected component of aFourier Transform representation of the light intensity distributionalong a single dimension of the entire optical image incident on adefined portion of the film for the frequency of vibration of thesubstrate at which the electrical signal is derived.
 5. A device as inclaim 1 wherein the means for vibrating the substrate causes thepropagation of surface waves in the substrate.
 6. A device as in claim 1wherein the film comprises substantially a photoconductive intrinsicsemiconductor.
 7. A device as in claim 1 wherein the substrate comprisessubstantially fused quartz and the film comprises substantiallypolycrystalline cadmium sulfide.
 8. A device for generating anelectrical signal representation of an optical image comprising: amedium comprising a film capable of undergoing time and space varyingstrain disturbances and having an electrical property wHich varies as afunction of an optical image incident on the film and as a function oftime and space varying strain disturbances in the film; means forcausing the incidence of an optical image on the film; means for causinga plurality of different time and space varying strain disturbances inthe film; and means for measuring said electrical property of the filmat a plurality of said different strain disturbances therein to derive aplurality of electrical signals representing said optical image.
 9. Adevice as in claim 8 wherein the medium comprises a substrateacoustically coupled with the film and capable of undergoing time andspace varying strain disturbances in acoustic coupling with the film.10. A device as in claim 9 wherein each of said electrical signals is afunction of the light intensity distribution along at least onedimension of the entire optical image incident on a defined portion ofthe film and on the strain disturbances in the film at which the signalis derived.
 11. A device as in claim 9 wherein said optical imageincident on the film is two-dimensional and wherein each of saidelectrical signals is a representation of the entire two-dimensionaloptical image incident on a selected portion of the film for the straindisturbances at which the signal is derived.
 12. A device as in claima 9wherein said electrical signals are selected components of a FourierTransform representation of the optical image.
 13. A device as in claim9 wherein each of said electrical signals is a selected component of aFourier Transform representation of the light distribution along atleast one selected dimension of the optical image.
 14. A device as inclaim 9 wherein each of the electrical signals is a selected componentof a Fourier Transform representation of the light distribution along asingle dimension of the optical image.
 15. A device as in claim 9wherein each of the electrical signals is a selected component of aFourier Transform representation of the light distribution along twononcongruent dimensions of the optical image incident on a selectedportion of the film.
 16. A device as in claim 8 wherein the means forcausing a plurality of different strain disturbances in the filmcomprises means for causing the propagation of a surface strain wave inthe film.
 17. A device as in claim 16 wherein the means for causingstrain disturbances comprises means for vibrating the film with acontrolled strain wave wherein the wave vector of the strain wave iscontrolled in at least two noncongruent dimensions.
 18. A device as inclaim 17 wherein each of said electrical signals is a component of aFourier Transform representation of the entire two-dimensional opticalimage incident on at least a portion of the film.
 19. A device as inclaim 16 wherein each of the electrical signals is a representation ofat least a component of the electrical conductivity of the film. 20.Method of obtaining an electrical signal representation of an opticalimage comprising: causing the incidence of an optical image on a mediumhaving an electrical property which varies as a function of an opticalimage incident on it and as a function of time and space varying surfacestrain wave disturbances in the medium; causing a plurality of differenttime and space varying surface strain wave disturances in the medium;and measuring said electrical property of the medium at a plurality ofsaid different surface strain wave disturbances in the medium to derivea plurality of electrical signals representing said optical image.
 21. Amethod as in claim 20 wherein each of said electrical signals is arepresentation of the light distribution along at least one dimension ofthe optical image incident on at least a portion of the medium.
 22. Amethod as in claim 20 wherein each of said electrical signals is arepresentation of the light distribution along two noncongruentdimensions of the optical image incidenT on at least a portion of themedium.
 23. A method as in claim 20 wherein each of said electricalsignals is a selected component of a Fourier Transform representation ofsaid optical image incident on at least a portion of the medium.
 24. Adevice as in claim 20 wherein each of said electrical signals is aselected component of a Fourier Transform representation of the lightdistribution along at least one dimension of the optical image incidenton a selected portion of the medium.
 25. A method as in claim 20 whereineach of said electrical signals is a selected component of a FourierTransform representation of the two-dimensional light distribution ofthe optical image incident on a selected portion of the medium.
 26. Amethod as in claim 20 including controlling the wave vector of thestrain wave in the medium in at least the two dimensions of the imageincident on the medium.
 27. A method as in claim 26 wherein each of saidelectrical signals is a selected component of a Fourier Transformrepresentation of the entire two-dimensional optical image incident on aselected portion of the medium.
 28. A method as in claim 20 wherein saidelectrical property of the medium is at least a selected component ofthe electrical conductivity of the medium.
 29. A device for generatingan electrical signal representation of an optical image comprising: asubstrate of a material capable of undergoing time and space varyingstrain disturbances and a film secured to a surface of the substrate toundergo strain disturbances therewith, said film having an electricalproperty which varies as a function of an optical image incident on thefilm and as a function of time and space varying strain disturbances inthe film; means for causing the incidence of an optical image on atleast a portion of the film; means for causing a plurality of differenttime and space varying strain disturbances in the film; and means formeasuring said electrical property of the film at a plurality of saiddifferent strain disturbances in the film to derive a plurality ofelectrical signals representing said optical image, said measuring meanscomprising an interdigitated plurality of first and second electricallyconductive contacts disposed over portions of the film in electricalcontact therewith, with the first contacts electrically connected toeach other and with the second contacts electrically connected to eachother, but with the first and second contacts spaced from each otheralong the film.
 30. A device as in claim 29 wherein the means forcausing strain disturbances causes the propagation of a surface strainwave along the surface of the substrate facing the film.
 31. A device asin claim 29 wherein each of said electrical signals is a selectedcomponent of a Fourier Transform representation of the optical imageincident on at least a portion of the film.
 32. A device as in claim 29wherein the substance is substantially fused quartz and the film issubstantially a photoconductive intrinsic semiconductor.
 33. A devicefor obtaining an electrical signal representation of an optical imagecomprising: a substrate of a material capable of undergoing time andspace varying strain disturbances and a photocathode film secured to asurface of the substrate to undergo said strain disturbances therewith;a photoelectron collector plate facing the photocathode film but spacedtherefrom to collect electron current emitted from the photocathode filmand means for establishing a potential difference between thephotocathode film and the photoelectron collector plate; means forcausing the incidence of an optical image on the photocathode film;means for causing a plurality of different time and space varying straindisturbances in the substrate and in the photocathode film; and meansfor measuring an electrical parameter of the current between thephotocathode film and the photoelectron collector plate at a pluralityoF said different strain disturbances in the photocathode film to derivea plurality of electrical signals representing the optical image.
 34. Adevice as in claim 33 wherein each of said electrical signals is aselected component of a Fourier Transform representation of the lightdistribution along at least one dimension of the optical image.
 35. Adevice as in claim 33 wherein each of a selected plurality of saidelectrical signals is a selected component of a Fourier Transformrepresentation of the entire two-dimensional optical image incident on aselected portion of the photocathode film.
 36. A device forreconstructing pictorial information represented by electrical signalseach of which is a selected component of a Fourier Transformrepresentation of said pictorial information comprising: a pair ofsubstantially parallel polarizing plates disposed at an angle ofapproximately 90* between their respective planes of polarization; aplate of elasto-optical material disposed intermediate the polarizingplates; means for propagating through said intermediate plate strainpatterns whose amplitudes and phases are governed by said electricalsignals; a light source for illuminating one side of the combination ofthe three plates; and means for displaying the light pattern emergingfrom the side of the combination of the plate which is opposite thelight source.